It was found that 96.2 per cent of the plague microbe cultures studied had a capacity to produce pesticin I on solid and liquid minimum synthetic nutrient media providing growth of the cultures. No pesticin III was detected in the spheroplasts of plague microbes, when they were grown on the synthetic nutrient medium, while on the complete nutrient medium the inhibition zones were observed which was indicative of pesticin III synthesis. The average value characterizing the titer of pesticin I produced by the rod-shaped forms of the plague microbe on the complete and synthetic nutrient media was 320. An increase in the titer of the spheroplast pesticin was shown, while the pesticin titer of the rod-shaped forms was low (1 : 20--1 : 320). As a result the average titer of the spheroplast pesticin was higher than that of pesticin of the rod-like forms. On the complete nutrient medium it was 1280 and on the synthetic medium it was 640.